When gas turbine or turboshaft engines are employed to drive a plant, machinery, or a vehicle, a high numerical reduction ratio is frequently needed because of the output speed of the turbine. In addition, power transmission of several thousands of horsepower is encountered in many applications. In the case of a stationary plant, or for marine applications, mechanical reliability can be readily achieved if the weight of the gearbox is not important. However, with propeller drives for aircraft or rotor drives for helicopters, weight of the gearbox is critically important. This requirement led to the widespread adoption of planetary or epicyclic gearboxes in flight applications. Planetary gearboxes achieve their weight advantage over simple gear trains of the same ratio by virtue of increasing the number of mesh points, and hence load-carrying gear engagements, in a given circumferential length of gearing.
With increasing scale and power transmission capacity, the weight of a gearbox increases approximately as a cube function of linear size because the steel elements of the gears span the entire radial distance from the center of rotation to the periphery of the largest gear, usually a ring gear. The tangential force resisting a torque is inversely proportional to the distance from the center of rotation, thus it is clear that whilst gear tooth loading from tangential force decreases with radius, weight increases disproportionately.
Efforts to improve the weight to torque/speed ratio are illustrated by the trend lines for the world population of aircraft and rotorcraft gearboxes in FIG. 10, in which weight on the vertical axis is plotted against a torque-speed equation on the horizontal axis. Here, the data were taken from approximately 70 different helicopters for a linear fit and included transmissions, rotor shaft(s), lubrication, and rotor brake. When corrected by a calendar year ‘technology factor’, the trend lines are remarkably linear (the technology factor takes into account the material, manufacturing, and lubricant improvements over a time span). For example, the top line in the graph of FIG. 10 is the trend for 1980 technology, while the middle line represents the corresponding trendline at a time 10 years later (i.e., for the year 2000 technology). A projected trendline for the year 2010 is depicted as the bottom line in FIG. 10. Therefore, desirable gearboxes will advantageously be situated below the 2000, and more preferably below the 2010 trendline with respect to their weight to torque/speed ratio. The performance of an aircraft, equipped with such gearbox, will therefore benefit by increased range or payload from the reduction of the empty weight fraction achieved by a lighter gear box arrangement.
Therefore, it should be readily apparent that the problem of gearbox specific weight per horsepower constantly recurs in aircraft designs and hence requires a solution. Consequently, there is still a need to provide improved gearboxes, and especially light-weight gearboxes for airplanes and other weight-critical uses.